High density limit in a Reversed Field Pinch

In Reversed Field Pinches (RFPs) a high density operational limit has been observed [1,2]. As for Tokamaks, in RFPs it can be well described by the Greenwald density[3] nG=Ip/pa2 (1020 m-3, MA, nG max line averaged density). Actually the Greenwald density has been transiently exceeded by pellet injection or careful control of the plasma-wall interaction in both the magnetic configurations, while Stellarators can more easily overcome the limit [4]. This contribution reports the experimental studies carried out in the RFP device RFX-mod to ascertain the physical mechanism underlying the density limit, aiming at discriminating if the Greenwald density represents an intrinsic boundary in the RFP operational space or if methods to overcome it may be found.